1. Field of the Invention
The present invention relates to materials supplied for fabricating single-crystal semiconductor, and more specifically, to a recharging or additional charging material supplied for fabricating single-crystal semiconductor by the Czhocralsky method (CZ method).
2. Description of Related Art
The substrate of semiconductor devices is generally of high purity single-crystal silicon which is, in general, fabricated by the CZ method. In the CZ method, polysilicon is charged in the crucible of a single-crystal pulling apparatus, in which the polysilicon is melted by heaters arranged around the crucible. In the method, a seed, which is appended to a seed holder, is immersed in the melted polysilicon and rotated in counter to or in the same direction as that of the crucible, the single-crystal is formed when the seed holder is pulled from the crucible.
As the required dimensions of the semiconductor wafer increased, large single-crystal silicon rods with a diameter greater than six inches have become main stream in the semiconductor industry recently. Therefore, the dimensions of the single-crystal silicon pulling apparatus have accordingly expanded and the processing amount per one batch process tends to increase. However, the time required to fabricate the single-crystal silicon also increases as the apparatus dimensions expand. Moreover, certain pre- and post-formation processes, such as melting more polysilicon in a larger crucible before the fabrication of single-crystal silicon and cooling the crucible to an appropriate temperature for cleaning after the fabrication of single-crystal silicon, require more processing time. All these requirements reduce the efficiency of single-crystal silicon fabrication.
The recharge method is one way to improve the fabrication efficiency of single-crystal silicon. This method recharges polysilicon in the crucible and melts the recharged polysilicon when the single-crystal silicon has been pulled out. When the recharge method is utilized, the cooling time and cleaning time of the elements and chambers in the pulling apparatus can be saved, and the quartz crucible, which must be replaced after formation of a single-crystal rod in the conventional fabrication method, can be reused, thus reducing the manufacturing cost. Moreover, an additional charge method can be utilized to feed additional polysilicon into the crucible when previous charged polysilicon is melted, thus increasing the melt amount and facilitating the fabrication of a longer single-crystal silicon rods of larger diameter.
In the recharge or additional charge process, as the polysilicon rods are provided for charge material, one end of each rod has a ring groove which can append to an suspending jig attached to a pulling axis or pulling cable of the single-crystal pulling apparatus, thus feeding the rods into the melt in the crucible. When the recharge or additional charge method is utilized to fabricate single-crystal rods of a large diameter, the amount of melted polysilicon should be increased, therefore larger or longer polysilicon rods are required.
However, certain problems arise as the larger or longer polysilicon rods are utilized. The problems are as follows.
1. The manufacturing cost of larger polysilicon rods is generally much higher because of their low growth rate and of poor yield due to cracks occurred during the growing process. Moreover, any valve through which the polysilicon rod is inserted into the pulling apparatus limits the dimensions of the rod. For example, the inert diameter of the gate valve connecting the main chamber to the pulling chamber and the diameter of the regulation pipe for conducting inert gas from upper portion of the pulling chamber to the main chamber are quite small, thus limiting the dimensions of the rods in the pulling apparatus. When two polysilicon rods of general dimension are combined to be melted in the crucible, they are constrained by the minimal diameter of the valves that they pass through.
2. If the large-dimensional polysilicon rods are not preprocessed by heating before being melted or not cooled after they melted with more times than the times usually required, the rods may break and drop in the crucible, thus increasing the time required to melt the rods and reducing the efficiency. For example, a polysilicon rod having a diameter of 120 mm and a length of 510 mm drops at a speed of 100 mm/sec to a distance of 20 mm from the melt surface in a crucible. The diameter of the crucible is 18 inches, and in the crucible remains 25 Kg of melt. If the polysilicon rod is pre-heated for 30 minutes, it will crack, and most of it will fall into the melt. For if only 30 mm of the polysilicon rod remains, and the remaining portion is pulled at a speed of 50 mm/sec from the chamber, the remaining portion has a high temperature of 504.degree. C., with a variation of 58.degree. C. between the highest and lowest temperatures.
3. As compared with common rods, the enlarged rods and rods of specific length are more expensive. Moreover, utilizing the enlarged rods, after charging in the crucible, leaves end portions which require a plenty of time to make good of them.
4. Not only novel apparatus for suspending the enlarged polysilicon rods, but cleaning and handling apparatus are required, thus increasing the manufacturing cost.
5. As the rod dimensions are expanded, the workability becomes extremely worse in making grooves before cleaning, the chamber cleaning, handling in resolving step and other processing steps cannot be efficiently carried out.